1. Field of the Invention
The present invention relates to a current sensing circuit and, more particularly, to a current sensing circuit for detecting a current flowing through a high-voltage/large-current power switch.
2. Description of the Related Art
For a synchronous switching DC/DC voltage regulator, an inductor current needs to be detected in magnitude and variation if a current-mode topology is configured as a mechanism of feedback control. Conventionally, a resistor is connected in series to the inductor and then a potential difference is caused across the resistor by the inductor current, which will provide the appropriate information regarding the magnitude and variation of the inductor current. However, the prior art must utilize the series-connected resistor with a result of the I2R power consumption. In an application of a large inductor current, the series-connected resistor must occupy a large surface area for satisfying the required current flow capacity under limitations determined by the nature of the semiconductor processing and materials, setting up a barrier to the development of a finer semiconductor chip. Moreover, an operational amplifier is necessary for retrieving the potential difference across the series-connected resistor, making the circuitry more complicated and reducing the operation speed.
FIG. 1 is a circuit block diagram showing a synchronous switching DC/DC voltage regulator provided with a conventional current sensing circuit. As shown in the figure, a high-side switch HS and a low-side switch LS are coupled in series between an input voltage source Vin and a ground potential. An inductor L has one terminal coupled to a node A between the high-side switch HS and the low-side switch LS, and the other terminal serving as an output terminal for supplying a regulated output voltage Vout. The output terminal may also be provided with an output capacitor Co for filtering ripples of the output voltage Vout. The high-side switch HS and the low-side switch LS are controlled by a high-side drive signal HD and a low-side drive signal LD, respectively, from a current-mode synchronous-switch control circuit 11. In the synchronous switching DC/DC voltage regulator, the high-side switch HS and the low-side switch LS are operated out of phase. When the high-side switch HS is turned ON and the low-side switch is turned OFF, the input voltage source Vin supplies energy to the inductor L, causing the inductor current IL to gradually increase. On the other hand, when the high-side switch HS is turned OFF and the low-side switch LS is turned ON, the energy stored in the inductor L is delivered to the output terminal as the output voltage Vout, causing the inductor current IL to gradually decrease.
Therefore, in a case that the high-side switch HS is implemented by a PMOS transistor and the low-side switch LS is implemented by an NMOS transistor, the high-side drive signal HD and the low-side drive signal LD are the pulse trains with the same phase. In a case that both of the high-side switch HS and the low-side switch LS are implemented by NMOS transistors, the high-side drive signal HD and the low-side drive signal LD are the pulse trains with 180 degrees out of phase therebetween. In addition, the high-side drive signal HD and the low-side drive signal LD are designed as non-overlapping pulse trains with a turn-on delay for preventing the high-side switch HS and the low-side switch LS from being simultaneously turned ON to erroneously short-circuit the input voltage source Vin and the ground potential.
For the current-mode feedback control mechanism, a resistor Rs is coupled in series to the inductor L for detecting the magnitude and variation of the inductor current IL. An operational amplifier 12 retrieves a potential difference across the series-connected resistor Rs caused by the inductor current IL, for generating a detection voltage Vs representative of the inductor current IL. Subsequently, the detection voltage Vs generated by the operational amplifier 12 is fed back to the current-mode synchronous-switch control circuit 11 for performing the current-mode control.
The series-connected resistor Rs is necessary in the prior art for detecting the inductor current IL, resulting in the IL2Rs power consumption. In an application where the inductor current IL should be made large, the series-connected resistor Rs must occupy a large surface area for satisfying the required current flow capacity under limitations determined by the nature of the semiconductor processing and materials. Moreover, the operational amplifier 12 for retrieving the potential difference across the series-connected resistor Rs makes the circuitry more complicated and reduces the operation speed.